Williams struggled with cocaine and alcohol abuse in the early '80s but was sober for 20 years ... with the exception of a stint in rehab in 2006.

One of Robin's last public appearances was at the San Francisco Zoo in June ... when the zoo named one of their howler monkeys "Robin" in his honor.

Robin's wife, Susan Schneider, says, "This morning, I lost my husband and my best friend, while the world lost one of its most beloved artists and beautiful human beings. I am utterly heartbroken."

Robin's last tweet was on July 31 ... wishing his daughter Zelda Rae a happy 25th birthday. He posted a #tbt instagram pic of himself holding her as a child. Zelda did not respond.

Late Monday evening Zelda tweeted a quote from French writer Antoine de Saint-Exupery ... "You alone will have the stars as no one else has them…In one of the stars I shall be living. In one of them I shall be laughing."